Abstract
The key feature of swelling clays such as montmorillonite, in contrast with the nonswelling clays, is their ability to adsorb water in the interlayer space. This interlayer water interacts with the interlayer cations or with the silicate layer surface inside the interlayer space, or with both. However, no direct experimental technique offers the possibility to determine separately these two contributions. In order to determine the hydration energy for interlayer alkali cations, we use a combination of electrostatic calculations of the surface energy and measurements of immersion heats in clays. The results show that Li+ and Na+ cations are characterized by a strongly exothermic hydration energy in the interlayer space, in contrast with K+, Rb+, and Cs+ which have a much lower hydration energy in the interlayer space. The extreme situation is that of Cs+, for which an endothermic hydration energy value is obtained. These trends are in good agreement with results from molecular modeling calculations and consistent with the evolution observed in the water adsorption isotherms. The hydration energy of the silicate layers was also calculated, and the total driving force for hydration in swelling clays could therefore be determined. For Li+- and Na+-montmorillonite, the hydration of cations is clearly the main contribution to the overall driving force for the hydration of clay. On the contrary, hydration of the silicate layers plays the most important role in the hydration of montmorillonite exchanged with the larger cations such as K+, Rb+, and Cs+. These results provide a physical basis for the differences observed in macroscopic swelling behavior between Li+- and Na+-montmorillonite, on one hand, and K+-, Rb+- and Cs+-montmorillonite, on the other hand.